Unmanned aerial vehicles (UAVs) have been used for a variety of purposes, such as reconnaissance or delivery of payloads. UAVs are powered aircraft with no human operator. They may fly autonomously or may be piloted remotely. Some UAVs are recoverable, while others are expendable after one flight. An example of a UAV is described in US Published Patent Application No. 2007-0018033, the disclosure of which is incorporated by reference herein.